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Structure and Function: The Joints

Joints are formed where two or more bones meet. Most joints, including those of the limbs, move freely and are known as synovial joints. They are lubricated by synovial fluid secreted by the joint lining. In contrast, semi-movable joints, such as those in the pelvis and spine, are less flexible but give greater stability. A few joints, such as those of the skull, are fixed and allow no movement.

Structure of a synovial joint

The bones of synovial joints are held together by ligaments that form a fibrous capsule. The synovial membrane lining the capsule secretes a lubricating fluid, and articular cartilage on the bone ends provides a smooth surface for movement.

Composition of articular cartilage

Articular cartilage is composed of cells (chondrocytes) located in cavities in a tough matrix of collagen, forming a smooth, flexible surface.

Types of joint

Illustrated here are six examples of synovial joints, each of a different type, and one fixed and one semi-movable joint. Synovial joints are classified according to how their articular surfaces (where bones meet) fit together and the movements each permits. The diagram accompanying each synovial joint illustrates its range of movement.

Fixed joint

In a fixed joint, the bones are bound together by fibrous tissue, allowing little or no movement. The fixed joints between the bones of the skull are called sutures.

Ball-and-socket joint

In a ball-and-socket joint, the ball-shaped end of one bone fits into a cup-shaped cavity in another, allowing movement in all directions. The shoulder and hip are ball-and-socket joints.

Ellipsoidal joint

The oval end of one bone fits into the oval cup of another in an ellipsoidal joint, allowing movement in most directions and limited rotation. The wrist is an ellipsoidal joint.

Hinge joint

The cylindrical surface of one bone fits into the groove of another to form a hinge joint. This type of joint either bends or straightens a limb. The knee, elbow, and finger joints are all examples of hinge joints.

Semi-movable joint

In a semi-movable joint, the articular surfaces are fused to a tough pad of cartilage that allows only a little movement. Examples are the pubic symphysis, which joins the front halves of the pelvis, and the joints of the spine.

Plane joint

In a plane joint, surfaces that are almost flat slide over each other, back and forth and sideways. Some joints in the foot and wrist are plane joints.

Saddle joint

Saddle-shaped bone ends that meet at right angles form a saddle joint. The bones can rotate a little and move sideways and back and forth. The body’s only saddle joint is at the base of the thumb.

Pivot joint

In a pivot joint, one bone rotates within a collar formed by another. The pivot joint between the atlas and the axis, the uppermost bones of the neck, allows the head to turn to either side.